I am designing a battery charger which gives 13.8V 7A or 27.6v 3.5A.
The topology is flyback and I decide the output(27V or 13V) with two
jumpers. the problem is that when I switch from 13.8 to 27.6V, VCC also
increases ()and I dont want VCC to increase above 30V because the FPS
of fairchild shuts down. So I put a 22V 1W zener diode and a resistor
to limit the current of zener on VCC circuitry. but those dissipate too
much power and zener live on the limits. Is there a nice method(fast
and cheap) to limit the voltage on VCC rather than using a
zener+resistor. thanks in advance